Plant Growth and Development

• Introduction

  • Definition of Growth
  • Definition of Development
  • Open form of growth (Meristems)
  • Indeterminate vs determinate growth

• Plant Growth

  • Measurable properties of growth
    • Fresh weight, dry weight, length, area, cell number, cell size
  • Phases of Growth
    • Meristematic phase (Constant division)
    • Elongation phase (Cell enlargement, vacuolation)
    • Maturation phase (Cell differentiation, maximum growth)
  • Growth Rates
    • Arithmetic growth ($L_t = L_0 + rt$)
    • Geometric growth ($W_1 = W_0 e^{rt}$)
    • Sigmoid growth curve (Lag, log/exponential, stationary phase)
  • Conditions for Growth
    • Water, oxygen, nutrients, temperature, light, gravity

• Differentiation, Dedifferentiation, and Redifferentiation

  • Differentiation (Specialization of cells, e.g., tracheary elements)
  • Dedifferentiation (Loss of capacity to divide to regain it, e.g., interfascicular cambium)
  • Redifferentiation (Loss of capacity to divide again to form specific tissues, e.g., secondary xylem/phloem)
  • Plasticity (Heterophylly in larkspur, buttercup)

• Plant Growth Regulators (PGRs) / Plant Hormones

  • Characteristics
    • Small, simple molecules of diverse chemical composition
    • Can be intracellular or intercellular
    • Plant Growth Promoters (Auxins, Gibberellins, Cytokinins)
    • Plant Growth Inhibitors (Abscisic acid, Ethylene)
  • Auxins
    • Discovery: Charles Darwin (canary grass coleoptiles)
    • Precursor: Indole-3-acetic acid (IAA)
    • Functions: Apical dominance, cell division, root initiation, parthenocarpy
  • Gibberellins (GAs)
    • Discovery: E. Kurosawa (bakane disease of rice)
    • Precursor: Gibberellic acid
    • Functions: Stem elongation, bolting in beet, breaking seed dormancy
  • Cytokinins
    • Discovery: Skoog and Miller (Kinetin from herring sperm DNA)
    • Functions: Cell division, overcomes apical dominance, delays leaf senescence
  • Ethylene
    • Gas PGR
    • Functions: Fruit ripening, horizontal growth of seedlings, triple response, abscission of flowers/leaves
  • Abscisic Acid (ABA)
    • Stress hormone
    • Functions: Seed dormancy, closure of stomata during water stress, abscission

• Photoperiodism

  • Definition (Response of plants to periods of day/night)
  • Classification of plants
    • Short Day Plants (SDP) (e.g., soybean, tobacco)
    • Long Day Plants (LDP) (e.g., radish, sugar beet)
    • Day-Neutral Plants (DNP) (e.g., tomato, sunflower)
  • Site of perception: Leaves (Phytohormone involvement)

• Vernalisation

  • Definition (Dependence of flowering on low temperature exposure)
  • Prevents premature reproductive development
  • Examples: Winter varieties of wheat, barley, rye